In a case of a tube-type pneumatic tire used for trucks, buses and the like, air impermeability of a tube has a large influence on the durability of the tire. Specifically, a tube having higher air impermeability exercises a positive influence on the durability of the tire.
Increasing the thickness of rubber of the tube is one of conceivable methods for imparting high air impermeability to the tube. However, increasing the thickness of the rubber of the tube increases an amount of rubber used for the tube, and thus increases the weight of the tire, and also causes increase in heat produced during running. The increase in the weight of the tire brings about a problem of increase in rolling resistance which causes higher fuel consumption. In addition, the increase in heat produced in the tube brings about a problem of deterioration of the durability of the tire.
On the other hand, thermoplastic resin has a characteristic that air impermeability thereof is better than that of rubber, although the elastic modulus thereof is larger than that of the rubber. Considering this, there comes an idea of forming the thermoplastic resin into a tire tube with a small thickness which is in inverse proportion to the elastic modulus. However, a tire tube made of the thermoplastic resin has a higher elastic modulus than that of the rubber, and a small thickness. For this reason, the tire tube made of the thermoplastic resin is highly likely to suffer cracks in its surface due to bending fatigue caused by contact with the inner circumferential surface of the tire. This may eventually lead to a puncture of the tube main body. Cracks in the tube main body are more likely to occur in a region where an uneven portion of a splice part of the tube main body is in contact with the inner circumferential surface of the tire tread part.
The following three cases are given as general generating mechanism of the occurrence of cracks in a tube made of such a thermoplastic resin. Prevention of the occurrence of cracks due to these cases has a direct relationship with the durability of the tube.    (1) Occurrence of cracks in the tube main body in a contact surface between the tube main body and the tire.    (2) Occurrence of cracks in the splice part of the two end portions of the tube main body.    (3) Occurrence of cracks in the joint section between the valve seat of the air injection valve and the tube main body.
On the other hand, as a proposal for improvement in the durability of the tube main body, Patent Document 1 has disclosed a rubber tire tube having a two-layer structure in which a first rubber layer and a second rubber layer are closely laminated together. The first rubber layer is made of 100 weight percent of butyl rubber, and the second rubber layer contains not less than 70 weight percent but not more than 100 weight percent of butyl rubber. In addition, the permanent tension set of the second rubber layer is set not less than 0.50 times but not more than 0.95 times as large as that of the first rubber layer.
In addition, as a tire tube whose durability is enhanced by alleviating its degradation with time during storage, Patent Document 2 has proposed a tire tube having a two-layer structure which includes: a main body layer formed of an elastic material; and a reinforcement layer formed of an elastic material different from the elastic material of the main body layer. The elastic material of the main body layer is a butyl-based rubber, and the elastic material of the reinforcement layer is a NR-SBR-BR-based rubber.
However, the tire tubes disclosed in Patent Documents 1 and 2 are still insufficient from a viewpoint of the improvement in the durability and air impermeability of the tube main body.
Furthermore, Patent Document 3 has proposed a tire inner tube including an air barrier layer of a laminated body which includes at least 25 layers of micro-layer composites each containing an elastomer blend and a thermoplastic resin with a high barrier property. In the case of this tube disclosed in Patent Document 3, however, the problem of enhancing the durability of the tube main body remains yet to be solved, although the air impermeability is enhanced.